Structure of the (Bi)carbonate Adlayer on Cu(100) Electrodes

Abstract (Bi)carbonate adsorption on Cu(100) in 0.1 M KHCO3 has been studied by in situ scanning tunneling microscopy. Coexistence of different ordered adlayer phases with (2 ×62 )R45° and (4×4) unit cells was observed in the double layer potential regime. The adlayer is rather dynamic and undergoes a reversible order‐disorder phase transition at 0 V vs. the reversible hydrogen electrode. Density functional calculations indicate that the adlayer consists of coadsorbed carbonate and water molecules and is strongly stabilized by liquid water in the adjacent electrolyte.


Experimental Section
The experiments were performed in electrochemical environment at room temperature using a PicoPlus STM (Agilent, Santa Clara, USA) and polypropylene-covered tungsten tips. Cu(100) single crystals (MaTecK) were used as working electrode. The copper single crystal was electropolished in 65%-70% orthophosphoric acid (Merck) at potentials between 1.8 and 2.8 V vs. a Pt wire counter electrode. At least three polishing steps of 10 s each were applied before every experiment. After polishing, the sample was rinsed with ultrapure water, then covered with a droplet of water or 0.1 M H2SO4 prepared from ultrapure sulfuric acid (Merck), and finally placed in the STM's electrochemical cell. Two platinum wire were used as pseudo-reference and counter electrodes in the in situ STM experiments. Cyclic voltammetry measurements after the experiments and measurement vs. a Standard Calomel Electrode (SCE) were used to calibrate the pseudo reference electrode. In order to avoid oxygen in the electrochemical cell, the electrochemical cell was kept under argon (5.0 N Ar) or CO2 (4.5 N CO2) atmosphere during the entire experiment. The STM data were recorded in constant current mode at tunneling currents of 0.05 nA to 0.7 nA and a bias potential of -0.05 V to -0.35V. The data analysis was carried out with the SPIP software (Image Metrology A/S, Horsholm, Denmark). In some of the images high-pass filtering was applied to increase contrast. Furthermore, in Fig. 1b the average function of SPIP was used to improve the contrast. The potassium bicarbonate electrolyte was prepared from KHCO3 (Sigma-Aldrich, 99.7%) and ultrapure water and was purified with pretreated Chelex 100 resin (Bio-Rad). [1] Prior to the purification, the Chelex was regenerated in two steps: First, the Chelex was stirred in 1 M HCl for 12 hours and rinsed with 5 l ultrapure water. Then, Chelex was stirred in 1 M KOH for 24 h at about 70 °C and rinsed with 8 l ultrapure water.    Figure S4. In situ STM image of the stripe-like (√  6√ )R45° structure in the double layer region (0.14 V, image high-pass filtered for better visibility), illustrating the coexistence of (√  6√ )R45° domains with different appearance. Specifically, the appearance along the stripes change from a double row feature to a more prominent single row (marked by black arrow) near a vertical running step in the center of the image.

Computational details
The simulations were run using ASE [2] with two different metal fcc (100) slabs, a regular (4 × 4 × 3 ) supercell and a 45° rotated (62 × 2 × 3) supercell. In both cases all the Cu layers were fixed to allow a faster force convergence. We test a series of surface compositions (pure CO3 coverages as well as mixed coverages of CO3 with HCO3, OH, and/or H2O) and several different starting configuration within each composition (see Table S1-S2). Note that in several cases different starting configurations converged into identical adlayer structures during the simulations (e.g., ID=2 and ID=11 in Figure S 10). The binding energies were calculated, using gas-phase CO2, H2O and H2, in the following way (illustrated for pure CO3 as an example): The electronic calculations were carried out at the generalized gradient approximation−density functional theory (GGA-DFT) level of theory, with the projector augmented wave method together with the BEEF-vdW functional [3] as implemented in the GPAW software. [4] A k-point sampling relevant for the specific structure, a grid spacing of 0.18 Å, and a vacuum of 10 Å were applied, and all of the structures were relaxed to a force below 0.05 eV/Å. This means that the binding energies were simulated in vacuum.
To estimate the effect of water solvation we utilize a Continuum Solvent Model (CSM) method as implemented in GPAW [5] , with standard parameters for water. Further, to simulate the observed STM images, we utilize the ASE and GPAW STM package. Structures with total energies, ensembles, and the plotting method are available on the webpage (https://nano.ku.dk/english/research/theoretical-electrocatalysis/katladb/structure-of-the-bicarbonate-adlayer-on-cu100-electrodes/), including the script to plot the data. We note that because of the high coverage of the adlayer structures, the adsorbate positions can only relax to a limited amount during the structure relaxation. Hence, the structures will be biased by our initial guesses, which we chose on the basis of the experimental observed STM images. For the (√  6√ )R45° structures our initial guesses are more similar to each other than for the (4 × 4) structures, resulting in lower energy differences. Specifically, we find for the 4 CO3 + 4 H2O configuration (Figure S11) 3 major energetic groups (Group 1: 0.1 eV, Group 2: 0.8 eV, and Group 3: 1.61 eV) and within each group multiple configurations. Group 1 corresponds to almost square arrangements of the adsorbates, Group 2 corresponds to row-like arrangements of carbonate and of hydrogen bonded water, and Group 3 corresponds to row-like arrangements in which no hydrogen bonds between the water exist.

Screening coverages
In this section data from the screening approach is shown.

Pourbaix analysis
In Figure S14 a schematic energy diagram used for the Pourbaix analysis is displayed. The purpose of this approach is to avoid calculating relevant species, such as HCO3 and CO3, in solution and instead employ a reference scheme. [6] From this energy diagram the Gibbs free energy of adsorption of carbonate from carbonate species in solution is given by: Here, is the number of proton-electron transfers, is Trassati's value for the absolute electrode potential of -4.4 V vs. SHE, is the Boltzmann constant, the temperature, is the pH, and is the work function of the adlayer structure obtained in the simulation. Note that in these vacuum simulations there is only one work function per adlayer structure. In the Pourbaix diagram we are plotting ΔG3 as a function of the work function, which represents the electrode potential. This is similar to assuming the adlayer energies are not affected by the electrode potential. Figure S16. Energy diagram to calculate the stability of adsorbed carbonate (*CO3) as a function of potential and pH.
Using the equilibrium constants: we can now derive the free energy of carbonate species in solution by four different equations as a function of pH: ∆ 1 is now given as a minimum of the four above and the thermodynamic values from Table S3: ∆ 1 = min[∆ ( 2 3 ), ∆ 1 ( 3 + ), ∆ 2 ( 3 + ), ∆ ( 3 + 2 )] − ( 2 ) − ( 2 ) (5) We now simulated the thermodynamic values of our coverage structure to get ∆ 2 : from which we obtain the Gibbs free energy of adsorption ΔG3 via eq. (2).